Thermionic emitter materials



y 1956 F. c. TODD ETAL 2,744,073

THERMIONIC EMITTER MATERIALS Filed Nov. 22, 1952 Fllfll Temperature I400I0 2O 30 40 5O 60 70 80 90 I00 I00 90 80 7o 60 5o 40 3o /o 06d 0 Per Gemof Gomponenls PULSE EMISSION CURRENT FROM MIXTURES 0F GADOL/IV/UM OXIDEWITH DYSPROS/UM OXIDE Fl 1;] E 6 1 I I O Temperature =I400 ,"0 5 oTemperofure I300 0 0 0o' 0 0 I0 v 20 40 1 6 0 8O 90 I00 I00 90 70 60 5040 30 20 I0 0-IVQO;

Per'OenI of Components PULSE EMISSION OF VARIOUS MIXTURES OF IVEODYMIUMOXIDE WITH GADOL/IVIUM OXIDE Emission Currem, Amp/0m V INVENTORS.Francis 0. Todd BY Eugene N. Wyler M We ATTORNEYS.

United States Patent THERMIONIC EMITTER MATERIALS Francis C. Todd,Columbus, and Eugene N. Wyler, Worthington, Ohio, assignors, by mesneassignments, to The Battelle Development Corporation, Columbus, Ohio, acorporation of Delaware Application November 22, 1952, Serial No. 3213851 Claim. (Cl. 252-521) This invention relates to electron emission and,particularly, to materials for thermionic emitters.

For many years work has been carried on to develop materials which wouldgive a high thermionic emission current when used as cathode surfaces invacuum tubes. In addition to a high thermionic emission, the materialsshould be stable at high temperatures in vacuua and should bemechanically strong. The materials should not be adversely aifected bypositive ion bombardment and should possess long life under theoperating conditions for which they are designed.

The oxide-coated cathode'which is now generally employed in vacuum tubesis not satisfactory for use in magnetrons or other vacuum tubes wherehigh emission current is required from the cathode. 'In pulse operation,this type of cathode sparks when high currents are drawn from it forlong pulses, and when heated to high temperatures it evaporatesexcessively, coating other parts of the tube. The thoriated tungstencathode is unsatisfactory for use in vacuum tubes where potentials aremore than about 10,000 volts. When the operating potentials exceed about10,000 volts, the thoriated tungsten cathode is deactivated by positiveion bombardment. Tungsten possesses the necessary electrical ruggednessrequired of a cathode and is not adversely affected by positive ionbombardment. Tungsten is not an efficient electron emitter, however, andit must be heated to very high temperatures before sufiicient electronemission can be obtained.

It has been discovered, as a part of this invention, that variousmixtures of the rare earth oxides give higher pulse emission than eitherof the pure components of the mixture. No sparkinghas been observed forcathodes coated with such mixtures at field strengths of the order of30,000 volts per centimeter. Even when subjected to severe arcing, thecathode surface suffers no apparent damage or deactivation. Withcontinued operation at temperatures of 1500 C. brightness the cathodecoatings of the rare earth oxide mixtures have shown no indications ofexcessive evaporation. The rare earth oxide mixtures have good chemicalstability in vacuua at high temperatures. All of these properties gotogether to produce a cathode coating with good emission properties anda long useful life. This type of material is particularly useful forapplication in magnetron cathodes where high electrical fields arerequired and where long life of the tube is desired. Cathodes of thistype can also be applied to any high-voltage, high-power tube wheresparking and excessive evaporation of the conventional cathode arelimiting factors.

Electron emitter materials according to the present invention compriserare-earth compounds (including compounds of lanthanum) in solidsolution. By mixing these compounds, the work function of the purecomponents of the mixture may be altered. The vapor pressure of themixtures is lower than that of the pure component of the mixture withthe highest vapor pressure. In addition, cathodes made from suchmixtures appear to be easier to activate than the pure components whichthey contain and are very resistant to emission poisoning.

It is a primary object of this invention, therefore, to providethermionic emitter materials comprising rareearth compounds (includinglanthanum compounds in such classification) in solid solution.

It is also an object of this invention to provide thermionic emittermaterials comprising solid solutions of rare-earth compounds to obtainthe foregoing and other desirable properties and advantages.

It is a further object of this invention to provide such solid solutionsin which the components are present in such proportions as to provide:(a) substantially maximum solubility of one existing crystal phase inanother crystal phase present in the solid solution; (12) substantiallythe lowest rate of diffusion and migration therein; (0) substantiallythe maximum distortion of the crystal lattice in the solution.

It is another important object of this invention to provide solidsolutions of rare-earth compounds wherein the pulse thermionic emissionof the solid solution exceeds that of any of its components.

Other objects and advantages will be apparent from the followingdetailed description.

In the drawings:

Fig. 1 comprises a graph, in rectangular coordinants, of pulsethermionic emissionat 1400 C. for various mix- .tures of dysprosiumoxide with gadolinium oxide; and

Fig. 2 comprises a graph, in rectangular coordinants, of pulsethermionic emission'at 1300 C. and at 1400- C. for various mixtures ofneodymium oxide with gadolinium oxide.

These figures illustrate advantages of this invention.

An electron emitter material according to thisinvention is composed of asolid solution of one component in one or more other components, or insolid solutions of the other components. When the materials are placedin solid solution, the pulse emission current is found to be greaterthan that for either of the two components, when only two components areused. If only a mixture of the two components were involved, then onewould expect the pulse emission current to be a linear function of thepercentage composition and lie between the values obtained for the twopure components separately. Since this is not the case, then one mightexpect that one of the components is going into solid solution with asingle-crystal phase formed by the two components. Ithas been discoveredas a part of this invention that the maximum pulse emission current isrealized when substantially the maximum solubility limit of one of thecomponents in a soild solution of the two components is reached. Thesame type of explanation applies where more than two components arepresent. The emission current would then be expected to vary betweenvalues obtained from solid solutions of the components and not from thevalues obtained from the pure components, but the discoveries of thisinvention show that greater emission currents can be obtained.

The materials which are used in mixtures of the rareearth compoundsshould go into solid solution one with the other, or others. The mostdesirable case is where the materials used have low vapor pressure andgive high thermionic emission current, i. e., materials with a low workfunction. Since quite a number of the good electron emitters in therare-earth series have a high vapor pressure, a good emitter of thistype may be mixed with a rare-earth compound which has a lower vaporpressure, but probably a higher work function, to provide an improvedemitter material. In this case, the vapor pressure of the solid solutionformed by such a mixture is lower than that of the component of themixture with the higher vapor pressure. According to Vegards law, thelattice constant of the solid solution of two components in each otheris given by a:a:-l-(ai-a2)A1. where a2 is the lattice dimension of therare-earth oxide with the smallest lattice spacing, tn is thecorresponding lattice dimension of the other rare-earth oxide, and A1 isthe fraction of the latter rare-earth oxide in the mixture. Since thevapor pressure is dependent upon the lattice spacing, this law providesa means of predicting the effect on the vapor pressure when solidsolution is formed.

The solid solution is formed by co-precipitating solutions of thematerials together in a suitable medium, such as oxalic acid, and theresulting precipitate is calcined at a suitable temperature, such asapproximates 1000 C. Since the rare-earth oxides are not particularlyunstable in air, no special precautions are necessary in their handlingexcept in the case of lanthanum oxide. Lanthanum must be handled as thecarbonate to avoid the formation of hydrates. This does not impose aserious handicap upon the use of a lanthanum oxide emitter since it maybe treated in the same manner as the ordinary present-day oxide cathode.

For the maximum efliciency a mixture should be chosen which gives themaximum emission current at the desired temperature of operation. Thecomposition of such a mixture may be determined experimentally byactually making emission measurements for various mixtures. It has beenfound for those mixtures upon which measurements have been made that themaximum pulse emission current is obtained near the low concentration ofone or the other of the components and not for the half-and-halfmixture. Since the position of the maximum in pulse emission current asa function of the proportions of the components present appears todepend upon the degree of solid solution, it is difficult to predict bycalculation where the peaks in pulse emission cur rent might occurbecause of the lack of information of the chemical and physicalproperties of the pure rareearth compounds. For those mixtures uponwhich measurements have been made, several peaks in emission currenthave been observed. Xray diffraction data have shown that these peaksoccur near the solubility limit of one or the other components in thecrystal phase of the solid solution of the components involved.

The crystal properties of the materials chosen for a mixture should beclose enough alike that a solid solution can be formed over a maximumrange of compositions. For a solid solution to occur for all proportionsof a mixture, it is necessary that the crystal structure be identical,and the dimensions of the lattice should be almost identical. Thecrystal dimensions and configuration may be determined by X-raydiffraction measurements.

After a mixture is once made, the material may be coated onto thecathode base by any of the commonly accepted techniques, such asspraying, for coating oxide cathodes. After the cathode is coated, nospecial activation, other than thorough outgassing by heating anddrawing emission current, is required. Mixtures of the rareearth oxidesdo not attack any of the commonly used cathode base materials.

The proportions of the mixtures of materials should be such that thesolubility limit of one of the components in a solid solution of theother components is reached. In some mixtures several peaks in theemission current may occur over a range of proportions of thecomponents. In other only one peak might occur over a range ofproportions.

Cathodes coated with mixtures of neodymium oxide with gadolinium oxide,dysprosium oxide with gadolinium oxide, and neodymium oxide with ceriumoxide, have been operated as thermionic emitters in an experimentaldiode. Gadolinium oxide with neodymium oxide, and gadolinium oxide withdysprosium oxide, were tested in several mixtures. The mixtures ofgadolinium oxide with neodymium oxide ranged in composition from zeropercent gadolinium oxide to zero per cent neodymium oxide. The mixturesof the gadolinium oxide with dysprosium oxide ranged in composition fromzero per cent gadolinium oxide to zero per cent dysprosium oxide. It wasfound that the maximum pulse emission current was obtained from acathode coated with a mixture of twentyfive per cent gadolinium oxidewith seventy-five per cent neodymium oxide by weight. The pulse emissioncurrent from this mixture was about six times greater than that from thepure component of the mixture with the higher emission current. Amixture of only one composition was tried for neodymium oxide withcerium oxide. The pulse emission current was much higher than thatobtained from either of the components of the mixtures. The maximumemission current for the mixtures of gadolinium oxide occurred when thecomposition approximated seventy-five per cent gadolinium oxide andtwenty-five per cent dysprosium oxide by weight.

These results are set forth in greater detail in Fig. l and Fig. 2 andin the following examples and tables. The thermionic emissionmeasurements in the examples were made by the following method.

After a coating has been prepared on a cathode, the cathode is insertedin a vacuum system. The system is exhausted to a pressure of about 10-mm. of mercury before heating the cathode is initiated. The temperatureof the cathode is slowly raised while outgassing occurs. After thetemperature of the cathode is raised to 1500" C. or more withoutsubstantially lowering the pressure, the D. C. voltage is applied acrossthe diode comprising the cathode and an auxiliary anode covering aplatinum anode. The voltage is then slowly raised until the emissioncurrent reaches the maximum safe value for the auxiliary anode. With thecathode still hot, the auxiliary anode is raised and the clean platinumanode is exposed for the remainder of the conditioning period and forthe emission measurements. The voltage is then raised slowly until thesaturation current is reached. This whole process may take as long as 48hours of operation before a steady pressure and a constant emissioncurrent are obtained. After this state of equilibrium is reached, theactual thermionic-emission measurements are initiated. Thesemeasurements are obtained by decreasing the voltage and the temperaturein steps, in order to maintain the cathode in a reasonably constantstate of activation. Alternate D. C. and pulse measurements are taken sothat the cathode is in the same state of activation for both types ofmeasurement. The usual procedure is to make at least three D. C.measurements and two pulse measurements. The saturation currents aredetermined from the characteristics curves. The results arereproducible.

EXAMPLE I Thermionic-emission measurements were made as described abovefor cathode coatings of neodymium oxide, of cerium oxide, and of a solidsolution comprising seventy-five per cent neodymium oxide andtwenty-five per cent cerium oxide. Table I lists the emission currentsat 1300 C. and at 1400 C., the work functions, and the Richardsonconstants for these emitter materials. From Table I it is apparent that,although cerium oxide by itself is an unsatisfactory material forthermionic emission, in fact so poor that no measurements could beobtained, the mixture of cerium oxide and neodymium oxide providesnearly twice as much pulse emission current at 1300" C. and three timesas much pulse emission current at 1400 C. as does neodymium oxide byitself.

Table IH lists the emission currents at 1300 C. and at 1400 C., the workfunctions, and the Richardson constants for these emitter materials.

Emission Constants .1 Electron Am Volts 2.18 1.99 1.83 2.08 Lowemission, unstable 15 two compounds in various proportions.

Table 1 Emission Current Amps/om yp Emission D. Pulse-.- D. 0----Pulse..-

Mixture EXAMPLE II The same type of thermionic emission measurements aswere made in the first example were made for cathodes coated withgadolinium oxide, with dysprosium oxide,

The pulse emission currents at 1300 C. and at 1400 C. listed in TableIII are plotted in Figand with six mixtures of the two compounds invarious 2 111- 2 of th drawing, From Table III and Figure 2,

proportions. The emission currents at 1400 0., the work it is apparentthat mixtures of the components provide functions, 4 and the RiChaIdSOHconstants, for these increased pulse emission, particularly the solidsolutions emitter materials are listed in Table II. The pulse emishavingf m about per cent to about per cent gadosion currents listed in TableII are plotted in Figure 1 linium oxide and the balance essentially allneodymium of the drawing. From Table II and Figure 1, it is ap- 25oxide, and those having from about 85 per cent to about parent thatincreased pulse emission is obtained with varilly ous mixtures of thecomponents, particularly with solid 81 0 5183 96 8556 8 O Q0 Q1L0 0 0 00 0 &&6 Q

Emission Constants Volts ZLLLZZLLZLZZZLZZ 015 0 2 7 3 5 62 2 277575 577637 2%221318%5212Q43 0 0 0 L0 5 1 0 LQLO 20 0 Amp/om 0 00 0 O L0 0 0 0 00 0 L0 0 Table 111 Type Emission 95 per cent gadolinium oxide and thebalance essentia all neodymium oxide.

Mixture The maximum pulse emission current appears to be obtained whenthe solubility limit of one of the components in the solid solution ofthe two or more components is A 2 at? E mission Constants Electron AmVolts Emission Table II solutions having from about 20 per cent to about30 per cent dysprosium oxide and the balance essentially all gadoliniumoxide.

mm l V. Tm E DPDP nmmm mmm m mmo mmmmm awman. t wnnnnnnw. m D%%%%%%Domnwwnm mhw hwwwm mmmwmmm ddddddm GoGGGG W%%%% %o imnmwzmo EXAMPLE IIIapproached. It is possible to prepare mixtures of various Thermionicemission measurements were made as in the rare-earth oxides which Willform solid solutions in which first two examples for cathodes coatedwith neodymium one of the crystal phases present approaches thesolubility oxide, with gadolinium oxide, and with mixtures of theselimit of one phase in another. The same mechanism holds for therare-earth borides, the rare-earth sulfides, the rareearth oxides withthe rare-earth borides or sulfides, or any combination of refractorycompounds of the rare earths suitable for cathode coating materials.

In addition to increasing the thermionic emission over that obtainedfrom the pure compounds used in the rareearth oxide mixtures, cathodesof this type have shown no temperature rise as a result of drawingpulsed emission current. Chemical stability beyond that of some of thepure components which are used in the mixtures has been observed. It ispredicted, because of the known chemical similarities, that mixtures ofrare-earth compounds of the series will also produce the same eifectthat is observed for mixtures of the rare-earths compounds of the 4series. Unavailability of rare earths of the 51 series has so farprevented any verification of this prediction.

The D. C.-emission data are included in the tables because these dataare more nearly in accord with what is normally to be expected from amixture of ingredients for thermionic emission. The D. C.-emissioncurrents for the mixtures tested lie predominantly between the D. C.-emission currents obtained with the individual compo nents separately,as would be expected. The results of the D. C.-emission measurements,therefore, tend to emphasize the unexpected character of the results forpulse emission of mixtures of rare-earth compounds according to thisinvention.

It is apparent from the foregoing disclosure that thermionic emittermaterials have been provided which comprise rare-earthcompounds(including lanthanum compounds in this classification) in solidsolutions wherein the pulse thermionic emission exceeds that of any ofthe components of the solid solution. It will be obvious to thoseskilled in the art that-various changes may be made without departingfrom the scope of this invention, which is not limited by the particulardescription above but may be defined in such broader terms as will comewithin the disclosure.

What is claimed is: e

A thermionic emitter material consisting essentially of a solid solutionof from about 15% to about gadolinium oxide and the balance essentiallyneodymium oxide.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Journal of the American Chemical Society v. 72, pages 1386-(1950). Article by McCullough. (Copy in Sci. Lib.)

Phys. Rev. 82, page 573 (1951). (Copy in Sci. Lib.)

